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Comparison of fracture characteristic of silicon nitride ceramics with and without second crystalline phase
Materials Letters 58 (2003) 74 – 79 www.elsevier.com/locate/matlet
Comparison of fracture characteristic of silicon nitride ceramics with and without second crystalline phase Byong-Taek Lee a,*, Byung-Dong Han b, Hai-Doo Kim b a b
Division of Advanced Materials Engineering, Kongju National University, 182 Sinkwan-dong, Kongju City, 314-701, South Korea Ceramic Materials Group, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon City, 641-010, South Korea Received 6 September 2002; accepted 16 April 2003
Abstract The comparison of microstructure and fracture characteristics of gas-pressure-sintered (GPS) Si3N4 ceramics with and without second crystalline phase at the junction regions was investigated by a combination of SEM, HRTEM and micro-indentation techniques. The bimodal microstructure composed of many large and fine rod-like Si3N4 grains was made by carbothermal reduction treatment (CRT). Most of triple regions were comprised of Y3AlSi2O7N2 as a second crystalline phase while all of grain boundaries and interfaces between Si3N4 and Y3AlSi2O7N2 phase were an amorphous layer of about 2 nm in thickness. Although a few large, rod-like Si3N4 grains were also observed in the sample without CRT, but most of junction regions existed with an amorphous phase. The value of fracture toughness in the sample with second crystalline phase at the junction regions by CRT was 6.6 MPa m1/2. This was slightly higher than the value without second crystalline phase (5.1 MPa m1/2). The main reason for the slight increase of the fracture toughness, even though showing well-developed bimodal microstructure, is the formation of the Y3AlSi2O7N2 phase at junction regions, which led the transgranular fracture of Si3N4 grains at local regions. D 2003 Elsevier B.V. All rights reserved. Keywords: Silicon nitride; Carbothermal reduction treatment; Microstructure; Fracture characteristic
1. Introduction To improve the fracture toughness of Si3N4 ceramics, there have been two typical approaches. The first one is the composite way, by dispersing a second phase such as SiCwhisker [1,2], ZrO2 [3,4] and TiN [5 –7] particles. In the composites, it has been known that the remarkable mismatching of lattice parameters and thermal expansion coefficients between matrix and second particles creates micro-cracking toughening. The second particles having high elastic modulus and large aspect ratio also give a remarkable shielding effect during the crack propagation by crack-bridging and crack-deflection mechanisms [2]. The other approach is a self-reinforcement by micro-structural control of Si3N4 matrix. In this particular case, the fracture toughness is closely related to the crystal structure of raw Si3N4 powders [8] and
* Corresponding author. Tel.: +82-416-850-8677; fax: +82-416-8582939. E-mail address: [email protected] (B.-T. Lee). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-577X(03)00418-X
size distribution of rod-like Si3N4 grains [9]. Up till now, to achieve the high-fracture toughened Si3N4 ceramics, it has been understood that the control of bimodal microstructure by addition of h-Si3N4 seed is more favorable than that of unimodal microstructure [10,11]. Recently, it has been reported that a new approach to obtain the bimodal structure of Si3N4 ceramic is due to the decreasing of oxygen content by carbothermal reduction treatment (CRT) [12]. On the other hand, the structure of triple junctions in the Si3N4 systems also can have an effect on the fracture characteristic, but there were no comparison reports on the effect of second phase at the triple junctions. The purpose of the present work is to investigate the effect of second crystalline phase at junction regions on the fracture behavior of gas-pressure-sintered (GPSed)-Si3N4 bodies, which focused on the characterization of grain morphologies and crystal structure of triple junctions by TEM and HRTEM techniques. In addition, by the observation of crack propagation made by indentation, the relationship between microstructure and fracture characteristic of GPSed-Si3N4 ceramics will be discussed.
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2. Experimental procedure For the synthesis of Si3N4 ceramic bodies, commercial Si3N4 and 6 wt.% Y2O3-1 wt.% Al2O3 powders as a sintering additive were used. In this case, 0% C, 0.5 wt.% carbon powders were added to make the changing of the crystal structure of triple junctions in the sintered bodies by CRT. The powders were mixed in ethanol using a planetary ball mill and then dried on a hot plate while stirring. The mixtures were pressed into pellets and then cold-isostatic pressed at 250 MPa. The CRT was carried out at 1450 jC for 10 h, and then sintered by GPS process at 1850 jC for 6 h under 2 MPa N2 gas pressure. The XRD and SEM were used to characterize the crystal structure and morphology of the GPSedSi3N4 bodies. The size distribution of Si3N4 grains was obtained by using an image analyzer. In order to characterize the macroscopic fracture behavior, cracks and fracture surfaces were made by Vickers indentation with a load of 30 kg. TEM samples for observation of microstructures and fracture behavior were prepared from thin slices, which were mechanically polished to a thickness of about 150 Am and punched out 3 mm in diameter by an ultrasonic cutter. After polishing to a mirror plane, indentations were made with a load of 50 g for 15 s by a Vickers indentor (Akashi MVKVL). Back of the indented plane was further polished to about 30 Am in thickness by a dimple grinder and finally milled with Ar ions at an accelerating voltage of 3 kV and the glancing angle of 25j. Carbon coating the sample, to prevent charge-up effect, was done prior to TEM observations. Using HRTEM (JEM-4000EX), the detailed internal microstructures were examined. The Vickers hardness was measured by indenting with a load of 1 kg. The fracture toughness K1C was calculated by indentation method using Evans equation with a load of 20 kg [13].
3. Results and discussion Typical SEM images showing the plasma-etched microstructure of Si3N4 bodies, seen in Fig. 1a and b, were made by gas-pressure-sintering (GPS) at 1850 jC without and with carbothermal-reduction treatment (CRT), respectively. This shows the various shapes of Si3N4 grains and the random distribution of the rod-like Si3N4 grains in the GPSed-bodies [14]. We can see that the morphologies of Si3N4 grains depend on the CRT, i.e., many large rod-like Si3N4 grains with 3– 4 Am in diameter were found in the sample accompanied with CRT. However, in the sample without CRT, the diameter of large rod-like grains was slightly smaller (2 –3 Am) and there was a decrease of their numbers. Fig. 2 shows TEM images of GPSed-bodies accompanied without (a) and with (b) CRT, respectively. Without the dependency of the CRT, the crystal structure of Si3N4 grains was h-type and the average grain size of fine Si3N4 grains was about 0.5 Am in diameter. The electron diffraction patterns of (p) and (q) were taken from the marked ‘p’
Fig. 1. SEM micrographs showing the plasma-etched microstructure of GPSed-Si3N4 bodies, (a) without CRT, (b) with CRT.
and ‘q’ region in Fig. 2a and b, respectively. From the observation of diffused-ring pattern (p) in the sample accompanied without CRT, it is confirmed that most of the junction regions were constructed with the amorphous phase. However, in the sample accompanied with CRT, most of the junction regions of Si3N4 grains were seen with some slightly different contrast because they exist with a crystalline phase, as can be seen in the electron diffraction pattern of ‘q’. Fig. 3 shows EDS results taken from the junction regions of the Si3N4 grains (marked p and q) and a Si3N4 grain (marked r) region in Fig. 2a and b. In the interior region of a Si3N4 grain, Si and N peaks were just detected, as can be seen in Fig. 3c. However, in the junction regions without dependency on the effect of CRT, the Y, Si, Al, O and N elements were detected as can be seen in Fig. 3a and b. This result indicates that junction regions were comprised Y – Si– Al – O – N components, although their compositions were slightly different depend on the CRT. One obvious point is that the intensity of oxygen peak in Fig. 3b accompanied with CRT was lower than that of without CRT (Fig. 3a). Thus, it is confirmed that oxygen contents at the triple regions were clearly decreased by the CRT. The decreasing of oxygen contents at the triple junctions promotes the crystallization of junction regions, and the change of microstructure at triple regions can give an effect on the fracture behavior of GPSed-Si3N4 bodies.
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Fig. 2. TEM micrographs of GPSed-Si3N4 bodies without CRT (a) and with CRT (b). Electron diffraction pattern ‘p’ and ‘q’ are taken from marked ‘p’ and ‘q’ in (a) and (b), respectively.
The area percentage of Si3N4 grains, according to the diameter of Si3N4 grains, is shown in Fig. 4. In the sample accompanied without CRT (Fig. 4a), large portions of the area were many fine, rod-like Si3N4 grains with sub-micrometer diameters while large, rod-like grains occupied a small percentage. However, in Fig. 4b, the sample accompanied with CRT, the peak was seen with a bimodal-like distribution rather than that of Fig. 4a, due to the increasing number of large, rod-like Si3N4 grains. Fig. 5 is a HRTEM image (a) and an enlarged image (b) of a rectangular region in Fig. 5a, which shows a triple region in the GPSed-Si3N4 body created without CRT. As can be frequently observed in the Si3N4 systems using Al2O3 – Y2O3 sintering additive [2,14], most of triple regions and grain boundaries existed with an amorphous phase, so that the HRTEM image of triple region showed short-range-ordered structure without lattice fringe, as seen in Fig. 5b. Fig. 6 shows a HRTEM image (a) and an
enlarged image (b) of a rectangular region in Fig. 6a, which is a triple region in the GPSed-Si3N4 body accompanied with CRT. In this case, most of the triple regions existed with a crystalline phase, as shown in Fig. 2b. From the analysis of electron diffraction patterns and EDS profiles, it is thought to be a Y3AlSi2O7N2 crystalline phase, in which ˚ . However, in this the inter-planar spacing d(110) was 2.02 A system, the amorphous phase was also observed at a few of the triple regions. In particular, an important observation is that the junction regions were not fully crystallized, i.e., the thin amorphous layers with about 2 nm in thickness were still observed at the interfaces between Si3N4 and Y3AlSi2 O7N2 phases. Material properties of GPSed-Si3N4 bodies dependent on the CRT are shown in Table 1. Although some changes of material properties were observed in both samples, we should consider that the fracture toughness of GPSed-body accompanied with CRT was not remarkably increased, i.e.,
Fig. 3. EDS profiles (a, b, c) taken from the ‘p’, ‘q’ and ‘r’ regions in Fig. 2, respectively.
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Fig. 6. HRTEM image (a) of GPSed-Si3N4 body accompanied with CRT and enlarged image (b) of a rectangle in (a).
Fig. 4. Area percentages of Si3N4 grains depend on the diameter of Si3N4 grains without CRT (a) and with CRT (b).
its value was 6.6 MPa m1/2, which is a slightly increased value compared to that of without CRT (5.1 MPa m1/2). To identify the fracture characteristic of GPSed-bodies having different microstructures according to the existence of second crystalline at junction regions, their fracture surfaces were observed by SEM (Fig. 7). Although the main fracture mode was an intergranular fracture in the sample without second crystalline phase at triple junctions, some transgranular fracture were also observed at local regions of GPSed-body accompanied with second crystalline, as indicated with arrowheads in Fig. 7b. However, since the sample
Fig. 5. HRTEM image (a) of GPSed-Si3N4 body accompanied without CRT and enlarged image (b) of a rectangle in (a).
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Table 1 Material properties of GPSed-Si3N4 bodies depend on the CRT process
Without carbothermal reduction (0 wt.% Carbon, 1850 jC 6 h) Carbothermal reduction (0.5 wt.% Carbon, 1850 jC 6 h)
Relative density (%)
Hardness (Hv)
Fracture toughness (MPa m1/2)
99.79
1605
5.1
99.99
1648
6.6
accompanied with second crystalline at junction regions contained many large, rod-like grains, the fracture surface was rougher than that without second crystalline. In general, in high-toughened Si3N4 ceramics, the main fracture mode was intergranular fracture, so that its morphology is rougher and sharper [2,4,7,14]. The observation of pulled-out, rodlike Si3N4 grains and their rough traces due to the bimodal microstructure indicates that the crack bridging and crackdeflection mechanisms were applied to inhibit crack growth. To understand the clear relationship between microstructures and crack path, micro-indented cracks were observed using the TEM [2,4,6].
Fig. 7. SEM fracture surfaces of GPSed-Si3N4 bodies accompanied without (a) and with (b) CRT.
Fig. 8. TEM micrographs showing crack propagation of GPSedSi3N4 bodies.
B.-T. Lee et al. / Materials Letters 58 (2003) 74–79
Fig. 8 is the TEM images of GPSed-bodies accompanied without (a) and with second crystalline phase (b and c) by CRT, showing the crack tip and wake zones made by microindentation. As indicated with arrows in Fig. 8a, the crack was propagated along grain boundaries and triple junctions since they existed as an amorphous phase, as shown in Fig. 5. Although some crack-deflection was observed weakly at the local region of crack wake and tip zones, the direction of the crack propagation was seen as a straight line since there is no notable toughening effect. In the image, a strong strain field contrast was observed at the crack tip zone due to the stress concentration. On the other hand, in the sample accompanied with second crystalline, although main cracks are propagated intergranularly, were also propagated transgranularly at the local regions, as indicated with arrowheads in Fig. 8b and c. The local transgranular fracture is mainly attributed the existence of crystalline Y3AlSi2O7N2 at junction regions, as shown in Fig. 6. The marked ‘P’ regions in Fig. 8b showed the cross-section of large, rod-like Si3N4 grains. In this case, we can understand the crack propagation by the stereographic imaging, i.e., when large, rod-like grains existed parallel to the crack direction, the crack was deflected at the large, rod-like Si3N4 grains and then branched. But, if large, rod-like Si3N4 grains existed perpendicular, as showed in Fig. 8c, some crack debonding and crack deflection phenomenon were observed at the marked ‘Q’ regions. In general, it has been known that the existence of an amorphous phase is detrimental to the high temperature mechanical properties. However, in a viewpoint of fracture toughening, an optimum content of amorphous phase at the grain boundaries and junction regions can give a beneficial effect on the fracture toughening because of easy leading of a typical intergranular fracture. From the observation of microstructures and crack propagation, it was confirmed that even though the bimodal microstructure was formed easily by carbon addition, the fracture toughness was not so remarkably due to the existence of Y3AlSi2O7N2 crystalline at junction regions of Si3N4 grains. Thus, to remarkably improve the fracture toughness of the GPSed-Si3N4 body, all of grain boundaries and junction regions must be kept with an amorphous phase as well as the formation of perfect bimodal microstructure. The facilitation of a typical intergranular fracture by bimodal microstructure, weak bonding of grain boundaries and junction regions can lead to the remarkable crack bridging and crack deflection mechanisms in the GPSed-Si3N4 systems.
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4. Conclusions In the present work, an evaluation study of microstructure and fracture characteristic of GPSed-Si 3 N 4 ceramics, depending on the existence of second crystalline at triple junctions, has been presented. In the sample accompanied with CRT, the bimodal-like microstructure was realized, containing many large and fine rod-like Si3N4 grains. Most of triple regions were existed with a Y3AlSi2O7N2 crystalline phase, while a thin amorphous phase was observed with about 2 nm in thickness at Si3N4 grain boundaries and interfaces between the Si3N4 and Y3AlSi2O7N2 phase. However, in the sample without CRT, the number of large, rodlike Si3N4 grains was lower than that of with CRT, and their junction regions existed with an amorphous phase. The main fracture mode was an intergranular fracture in both samples, although the fracture surface of the sample accompanied with second crystalline phase at the triple junctions was rougher than that of without second crystalline. In the sample accompanied with second crystalline phase by CRT, local transgranular fracture was observed, due to the existence of crystalline phase at grain junctions. The values of fracture toughness in the samples, with and without second crystalline at triple junctions, were 6.6 and 5.1 MPa m1/2, respectively, in which the observation of crack deflection and crack bridging mechanisms was due to the existence of large, rodlike Si3N4 grains.
References [1] M.J. Hoffmann, A. Nagel, P. Greil, G. Petzow, J. Am. Ceram. 72 (1989) 765. [2] B.T. Lee, K. Hiraga, Mater. Trans., JIM 34 (1993) 930. [3] L.K.L. Falk, K. Rundgren, J. Am. Ceram. 75 (1992) 28. [4] B.T. Lee, T. Koyama, A. Nishiyama, K. Hiraga, Scrip. Met. et Mater. 32 (1995) 1073. [5] G. Hillinger, V. Hlavacek, J. Am. Ceram. 78 (1995) 495. [6] B.T. Lee, S. Hayashi, T. Hirai, K. Hiraga, Mater. Trans., JIM 34 (1993) 573. [7] B.T. Lee, H.D. Kim, J. Mater. Sci. 34 (1999) 6169. [8] D.D. Lee, S.J. Kang, D.N. Yoon, J. Am. Ceram. 81 (1998) 803. [9] K. Matsuhiro, T. Takahashi, Ceram. Eng. Sci. Proc. 10 (1989) 807. [10] M. Mitomo, S. Uenosono, J. Am. Ceram. Soc. 75 (1992) 103. [11] N. Hirosaki, Y. Akimune, M. Mitomo, J. Am. Ceram. Soc. 77 (1994) 1093. [12] H.D. Kim, B.D. Han, D.S. Park, B.T. Lee, P. Becher, J. Am. Ceram. Soc. 85 (2002) 245. [13] A.G. Evans, E.A. Charles, J. Am. Ceram. Soc. 59 (1976) 371. [14] B.T. Lee, J.H. Yoo, H.D. Kim, Mater. Trans., JIM 41 (2000) 312.
Comparison of fracture characteristic of silicon nitride ceramics with and without second crystalline phase Byong-Taek Lee a,*, Byung-Dong Han b, Hai-Doo Kim b a b
Division of Advanced Materials Engineering, Kongju National University, 182 Sinkwan-dong, Kongju City, 314-701, South Korea Ceramic Materials Group, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon City, 641-010, South Korea Received 6 September 2002; accepted 16 April 2003
Abstract The comparison of microstructure and fracture characteristics of gas-pressure-sintered (GPS) Si3N4 ceramics with and without second crystalline phase at the junction regions was investigated by a combination of SEM, HRTEM and micro-indentation techniques. The bimodal microstructure composed of many large and fine rod-like Si3N4 grains was made by carbothermal reduction treatment (CRT). Most of triple regions were comprised of Y3AlSi2O7N2 as a second crystalline phase while all of grain boundaries and interfaces between Si3N4 and Y3AlSi2O7N2 phase were an amorphous layer of about 2 nm in thickness. Although a few large, rod-like Si3N4 grains were also observed in the sample without CRT, but most of junction regions existed with an amorphous phase. The value of fracture toughness in the sample with second crystalline phase at the junction regions by CRT was 6.6 MPa m1/2. This was slightly higher than the value without second crystalline phase (5.1 MPa m1/2). The main reason for the slight increase of the fracture toughness, even though showing well-developed bimodal microstructure, is the formation of the Y3AlSi2O7N2 phase at junction regions, which led the transgranular fracture of Si3N4 grains at local regions. D 2003 Elsevier B.V. All rights reserved. Keywords: Silicon nitride; Carbothermal reduction treatment; Microstructure; Fracture characteristic
1. Introduction To improve the fracture toughness of Si3N4 ceramics, there have been two typical approaches. The first one is the composite way, by dispersing a second phase such as SiCwhisker [1,2], ZrO2 [3,4] and TiN [5 –7] particles. In the composites, it has been known that the remarkable mismatching of lattice parameters and thermal expansion coefficients between matrix and second particles creates micro-cracking toughening. The second particles having high elastic modulus and large aspect ratio also give a remarkable shielding effect during the crack propagation by crack-bridging and crack-deflection mechanisms [2]. The other approach is a self-reinforcement by micro-structural control of Si3N4 matrix. In this particular case, the fracture toughness is closely related to the crystal structure of raw Si3N4 powders [8] and
* Corresponding author. Tel.: +82-416-850-8677; fax: +82-416-8582939. E-mail address: [email protected] (B.-T. Lee). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-577X(03)00418-X
size distribution of rod-like Si3N4 grains [9]. Up till now, to achieve the high-fracture toughened Si3N4 ceramics, it has been understood that the control of bimodal microstructure by addition of h-Si3N4 seed is more favorable than that of unimodal microstructure [10,11]. Recently, it has been reported that a new approach to obtain the bimodal structure of Si3N4 ceramic is due to the decreasing of oxygen content by carbothermal reduction treatment (CRT) [12]. On the other hand, the structure of triple junctions in the Si3N4 systems also can have an effect on the fracture characteristic, but there were no comparison reports on the effect of second phase at the triple junctions. The purpose of the present work is to investigate the effect of second crystalline phase at junction regions on the fracture behavior of gas-pressure-sintered (GPSed)-Si3N4 bodies, which focused on the characterization of grain morphologies and crystal structure of triple junctions by TEM and HRTEM techniques. In addition, by the observation of crack propagation made by indentation, the relationship between microstructure and fracture characteristic of GPSed-Si3N4 ceramics will be discussed.
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2. Experimental procedure For the synthesis of Si3N4 ceramic bodies, commercial Si3N4 and 6 wt.% Y2O3-1 wt.% Al2O3 powders as a sintering additive were used. In this case, 0% C, 0.5 wt.% carbon powders were added to make the changing of the crystal structure of triple junctions in the sintered bodies by CRT. The powders were mixed in ethanol using a planetary ball mill and then dried on a hot plate while stirring. The mixtures were pressed into pellets and then cold-isostatic pressed at 250 MPa. The CRT was carried out at 1450 jC for 10 h, and then sintered by GPS process at 1850 jC for 6 h under 2 MPa N2 gas pressure. The XRD and SEM were used to characterize the crystal structure and morphology of the GPSedSi3N4 bodies. The size distribution of Si3N4 grains was obtained by using an image analyzer. In order to characterize the macroscopic fracture behavior, cracks and fracture surfaces were made by Vickers indentation with a load of 30 kg. TEM samples for observation of microstructures and fracture behavior were prepared from thin slices, which were mechanically polished to a thickness of about 150 Am and punched out 3 mm in diameter by an ultrasonic cutter. After polishing to a mirror plane, indentations were made with a load of 50 g for 15 s by a Vickers indentor (Akashi MVKVL). Back of the indented plane was further polished to about 30 Am in thickness by a dimple grinder and finally milled with Ar ions at an accelerating voltage of 3 kV and the glancing angle of 25j. Carbon coating the sample, to prevent charge-up effect, was done prior to TEM observations. Using HRTEM (JEM-4000EX), the detailed internal microstructures were examined. The Vickers hardness was measured by indenting with a load of 1 kg. The fracture toughness K1C was calculated by indentation method using Evans equation with a load of 20 kg [13].
3. Results and discussion Typical SEM images showing the plasma-etched microstructure of Si3N4 bodies, seen in Fig. 1a and b, were made by gas-pressure-sintering (GPS) at 1850 jC without and with carbothermal-reduction treatment (CRT), respectively. This shows the various shapes of Si3N4 grains and the random distribution of the rod-like Si3N4 grains in the GPSed-bodies [14]. We can see that the morphologies of Si3N4 grains depend on the CRT, i.e., many large rod-like Si3N4 grains with 3– 4 Am in diameter were found in the sample accompanied with CRT. However, in the sample without CRT, the diameter of large rod-like grains was slightly smaller (2 –3 Am) and there was a decrease of their numbers. Fig. 2 shows TEM images of GPSed-bodies accompanied without (a) and with (b) CRT, respectively. Without the dependency of the CRT, the crystal structure of Si3N4 grains was h-type and the average grain size of fine Si3N4 grains was about 0.5 Am in diameter. The electron diffraction patterns of (p) and (q) were taken from the marked ‘p’
Fig. 1. SEM micrographs showing the plasma-etched microstructure of GPSed-Si3N4 bodies, (a) without CRT, (b) with CRT.
and ‘q’ region in Fig. 2a and b, respectively. From the observation of diffused-ring pattern (p) in the sample accompanied without CRT, it is confirmed that most of the junction regions were constructed with the amorphous phase. However, in the sample accompanied with CRT, most of the junction regions of Si3N4 grains were seen with some slightly different contrast because they exist with a crystalline phase, as can be seen in the electron diffraction pattern of ‘q’. Fig. 3 shows EDS results taken from the junction regions of the Si3N4 grains (marked p and q) and a Si3N4 grain (marked r) region in Fig. 2a and b. In the interior region of a Si3N4 grain, Si and N peaks were just detected, as can be seen in Fig. 3c. However, in the junction regions without dependency on the effect of CRT, the Y, Si, Al, O and N elements were detected as can be seen in Fig. 3a and b. This result indicates that junction regions were comprised Y – Si– Al – O – N components, although their compositions were slightly different depend on the CRT. One obvious point is that the intensity of oxygen peak in Fig. 3b accompanied with CRT was lower than that of without CRT (Fig. 3a). Thus, it is confirmed that oxygen contents at the triple regions were clearly decreased by the CRT. The decreasing of oxygen contents at the triple junctions promotes the crystallization of junction regions, and the change of microstructure at triple regions can give an effect on the fracture behavior of GPSed-Si3N4 bodies.
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Fig. 2. TEM micrographs of GPSed-Si3N4 bodies without CRT (a) and with CRT (b). Electron diffraction pattern ‘p’ and ‘q’ are taken from marked ‘p’ and ‘q’ in (a) and (b), respectively.
The area percentage of Si3N4 grains, according to the diameter of Si3N4 grains, is shown in Fig. 4. In the sample accompanied without CRT (Fig. 4a), large portions of the area were many fine, rod-like Si3N4 grains with sub-micrometer diameters while large, rod-like grains occupied a small percentage. However, in Fig. 4b, the sample accompanied with CRT, the peak was seen with a bimodal-like distribution rather than that of Fig. 4a, due to the increasing number of large, rod-like Si3N4 grains. Fig. 5 is a HRTEM image (a) and an enlarged image (b) of a rectangular region in Fig. 5a, which shows a triple region in the GPSed-Si3N4 body created without CRT. As can be frequently observed in the Si3N4 systems using Al2O3 – Y2O3 sintering additive [2,14], most of triple regions and grain boundaries existed with an amorphous phase, so that the HRTEM image of triple region showed short-range-ordered structure without lattice fringe, as seen in Fig. 5b. Fig. 6 shows a HRTEM image (a) and an
enlarged image (b) of a rectangular region in Fig. 6a, which is a triple region in the GPSed-Si3N4 body accompanied with CRT. In this case, most of the triple regions existed with a crystalline phase, as shown in Fig. 2b. From the analysis of electron diffraction patterns and EDS profiles, it is thought to be a Y3AlSi2O7N2 crystalline phase, in which ˚ . However, in this the inter-planar spacing d(110) was 2.02 A system, the amorphous phase was also observed at a few of the triple regions. In particular, an important observation is that the junction regions were not fully crystallized, i.e., the thin amorphous layers with about 2 nm in thickness were still observed at the interfaces between Si3N4 and Y3AlSi2 O7N2 phases. Material properties of GPSed-Si3N4 bodies dependent on the CRT are shown in Table 1. Although some changes of material properties were observed in both samples, we should consider that the fracture toughness of GPSed-body accompanied with CRT was not remarkably increased, i.e.,
Fig. 3. EDS profiles (a, b, c) taken from the ‘p’, ‘q’ and ‘r’ regions in Fig. 2, respectively.
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Fig. 6. HRTEM image (a) of GPSed-Si3N4 body accompanied with CRT and enlarged image (b) of a rectangle in (a).
Fig. 4. Area percentages of Si3N4 grains depend on the diameter of Si3N4 grains without CRT (a) and with CRT (b).
its value was 6.6 MPa m1/2, which is a slightly increased value compared to that of without CRT (5.1 MPa m1/2). To identify the fracture characteristic of GPSed-bodies having different microstructures according to the existence of second crystalline at junction regions, their fracture surfaces were observed by SEM (Fig. 7). Although the main fracture mode was an intergranular fracture in the sample without second crystalline phase at triple junctions, some transgranular fracture were also observed at local regions of GPSed-body accompanied with second crystalline, as indicated with arrowheads in Fig. 7b. However, since the sample
Fig. 5. HRTEM image (a) of GPSed-Si3N4 body accompanied without CRT and enlarged image (b) of a rectangle in (a).
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Table 1 Material properties of GPSed-Si3N4 bodies depend on the CRT process
Without carbothermal reduction (0 wt.% Carbon, 1850 jC 6 h) Carbothermal reduction (0.5 wt.% Carbon, 1850 jC 6 h)
Relative density (%)
Hardness (Hv)
Fracture toughness (MPa m1/2)
99.79
1605
5.1
99.99
1648
6.6
accompanied with second crystalline at junction regions contained many large, rod-like grains, the fracture surface was rougher than that without second crystalline. In general, in high-toughened Si3N4 ceramics, the main fracture mode was intergranular fracture, so that its morphology is rougher and sharper [2,4,7,14]. The observation of pulled-out, rodlike Si3N4 grains and their rough traces due to the bimodal microstructure indicates that the crack bridging and crackdeflection mechanisms were applied to inhibit crack growth. To understand the clear relationship between microstructures and crack path, micro-indented cracks were observed using the TEM [2,4,6].
Fig. 7. SEM fracture surfaces of GPSed-Si3N4 bodies accompanied without (a) and with (b) CRT.
Fig. 8. TEM micrographs showing crack propagation of GPSedSi3N4 bodies.
B.-T. Lee et al. / Materials Letters 58 (2003) 74–79
Fig. 8 is the TEM images of GPSed-bodies accompanied without (a) and with second crystalline phase (b and c) by CRT, showing the crack tip and wake zones made by microindentation. As indicated with arrows in Fig. 8a, the crack was propagated along grain boundaries and triple junctions since they existed as an amorphous phase, as shown in Fig. 5. Although some crack-deflection was observed weakly at the local region of crack wake and tip zones, the direction of the crack propagation was seen as a straight line since there is no notable toughening effect. In the image, a strong strain field contrast was observed at the crack tip zone due to the stress concentration. On the other hand, in the sample accompanied with second crystalline, although main cracks are propagated intergranularly, were also propagated transgranularly at the local regions, as indicated with arrowheads in Fig. 8b and c. The local transgranular fracture is mainly attributed the existence of crystalline Y3AlSi2O7N2 at junction regions, as shown in Fig. 6. The marked ‘P’ regions in Fig. 8b showed the cross-section of large, rod-like Si3N4 grains. In this case, we can understand the crack propagation by the stereographic imaging, i.e., when large, rod-like grains existed parallel to the crack direction, the crack was deflected at the large, rod-like Si3N4 grains and then branched. But, if large, rod-like Si3N4 grains existed perpendicular, as showed in Fig. 8c, some crack debonding and crack deflection phenomenon were observed at the marked ‘Q’ regions. In general, it has been known that the existence of an amorphous phase is detrimental to the high temperature mechanical properties. However, in a viewpoint of fracture toughening, an optimum content of amorphous phase at the grain boundaries and junction regions can give a beneficial effect on the fracture toughening because of easy leading of a typical intergranular fracture. From the observation of microstructures and crack propagation, it was confirmed that even though the bimodal microstructure was formed easily by carbon addition, the fracture toughness was not so remarkably due to the existence of Y3AlSi2O7N2 crystalline at junction regions of Si3N4 grains. Thus, to remarkably improve the fracture toughness of the GPSed-Si3N4 body, all of grain boundaries and junction regions must be kept with an amorphous phase as well as the formation of perfect bimodal microstructure. The facilitation of a typical intergranular fracture by bimodal microstructure, weak bonding of grain boundaries and junction regions can lead to the remarkable crack bridging and crack deflection mechanisms in the GPSed-Si3N4 systems.
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4. Conclusions In the present work, an evaluation study of microstructure and fracture characteristic of GPSed-Si 3 N 4 ceramics, depending on the existence of second crystalline at triple junctions, has been presented. In the sample accompanied with CRT, the bimodal-like microstructure was realized, containing many large and fine rod-like Si3N4 grains. Most of triple regions were existed with a Y3AlSi2O7N2 crystalline phase, while a thin amorphous phase was observed with about 2 nm in thickness at Si3N4 grain boundaries and interfaces between the Si3N4 and Y3AlSi2O7N2 phase. However, in the sample without CRT, the number of large, rodlike Si3N4 grains was lower than that of with CRT, and their junction regions existed with an amorphous phase. The main fracture mode was an intergranular fracture in both samples, although the fracture surface of the sample accompanied with second crystalline phase at the triple junctions was rougher than that of without second crystalline. In the sample accompanied with second crystalline phase by CRT, local transgranular fracture was observed, due to the existence of crystalline phase at grain junctions. The values of fracture toughness in the samples, with and without second crystalline at triple junctions, were 6.6 and 5.1 MPa m1/2, respectively, in which the observation of crack deflection and crack bridging mechanisms was due to the existence of large, rodlike Si3N4 grains.
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